1. Field of the Invention
The present invention relates generally to a picture-in-picture television which can reproduce two different pictures simultaneously on a common display screen (such as a cathode ray tube).
2. Description of the Prior Art
Conventionally, picture-in-picture (P in P) television receivers have been well known as shown in FIG. 1, in which a B-channel picture (referred to as subpicture) is displayed being inserted at a predetermined area of an A-channel picture (referred to as main picture) by reducing both the lateral and vertical size of the subpicture to that of 1/3, for instance, of the main picture to reproduce both the main picture and subpicture simultaneously. In these P in P television receivers, if the subpicture is reduced to 1/3, for instance, first the subpicture signals are A-D converted, and then the vertical scanning lines are extracted in a ratio of 1 out of 3 before being written in a picture memory. In this case, the subpicture signals are written in the memory in response to a clock signal synchronized with the synchronizing signal of the subpicture signals, while the subpicture signals are read at a speed three times higher than that in writing the signals in response to a clock signal synchronized with the synchronizing signal of the main picture signal.
In the above-mentioned P in P television receiver, since the sampling frequency on the subpicture side is different from that on the main picture side, it is inevitably necessary to convert the sampling frequency somewhere in the circuit of the P in P television receiver.
In more detail, a subpicture signal is converted from an analog signal to a digital signal, stored in a memory via a write process circuit, read from the memory, and converted from the digital signal to an analog signal. During these operations, the subpicture signal is composed of a plurality of bit signals arranged in series. However, since the time during which variation in signal data becomes stable is different among these bit signals, there exists a problem in that there exists unstable data duration intervals, until the whole bit data become stable.